First, thank you much for the guide! I've decided to play an Investigator this weekend for our 3rd attempt into the Jade Regent AP, and had zero idea what I was doing having never played a Alchemist or a Rogue. So this was very helpful in guiding me to where I needed to go.
Second, I plan on doing the dex based TWF Brains build, and was wondering why you chose to go extend potion at 3rd level as opposed to mutagen or enhance potion? I understand we need extend eventually for eternal potion, but why not wait until later on to get it? The bonuses from Mutagen seem really helpful, especially early on while extend potion really only seems good for those potions you buy.
I am still newish to this, so there's probably something painfully obvious that I'm missing, but I guess the core of my question is would mutagen instead of extend potion at 3rd level be just as viable?